Usually, a resistive bolometric detector measures the power of an incident radiation in the infrared range. For this purpose, it comprises an absorbing resistive element, which converts the light flow into a heat flow, which generates a temperature rise of said element with respect to a reference temperature. This temperature increase then induces a variation of the electric resistance of the thermometric element, thus causing voltage or current variations thereacross. Such electric variations form the signal delivered by the sensor.
However, the temperature of the absorbing element is usually greatly dependent on its environment, and especially on the temperature of the substrate which comprises the electronic read circuit. To desensitize as much as possible the absorbing element from its environment, and thus increase the detector sensitivity, the absorbing element is generally thermally insulated from the substrate.
FIG. 1 is a simplified perspective view of an elementary resistive bolometric detector 10 of the state of the art illustrating the thermal insulation principle. Such an elementary detector, appearing in the described example in the form of a suspended membrane, conventionally belongs to a one- or two-dimensional array of elementary detectors.
Detector 10 comprises a thin membrane 12 absorbing the incident radiation, suspended above a substrate—support 14 via two conductive anchoring nails 16, having said membrane attached thereto by two thermal insulation arms 18. Membrane 12 usually comprises a stack of a dielectric layer and of a metal layer. The metal layer ensures the absorption function and the dielectric layer electrically insulates the metal layer from the thermometric element.
A thin layer 20 of resistive thermometric material is further deposited at the center of membrane 12, especially a layer made of a semiconductor material, such as highly or weakly resistive polysilicon or amorphous p- or n-type silicon, or a vanadium oxide (V2O5, VO2) formed in a semiconductor phase.
Finally, substrate—support 14 comprises an electronic circuit integrated on a silicon wafer, usually known as a “read circuit”. The read circuit comprises, on the one hand, the excitation and read elements of thermometric element 20, and on the other hand the multiplexing components which enable to serialize the signals originating from the different thermometric elements present in the detector array.
In operation, membrane 12 heats up under the effect of an incident electromagnetic radiation and the generated thermal power is transmitted to thermometric material layer 20. Periodically, the read circuit arranged in substrate 14 biases thermometric element 20 by submitting nails 16 to a bias voltage, and collects the current flowing through thermometric element 20 to deduce therefrom a variation of its resistance, and thus the incident radiation having caused said variation.
For brevity, the arrangement and the operation of such a detector being conventional, it will not be explained in further detail. It should however be noted that membrane 12 fulfils, in addition to the thermal insulation function, three main functions: an antenna function to receive the radiation, a function of conversion of the received electromagnetic power into thermal power, and a function of thermometric measurement of the generated thermal power. Since it is used as an antenna, membrane 12 has dimensions which are accordingly selected to be of the same order of magnitude as the wavelength of the radiation to be measured.
Now, in the terahertz range, wavelengths may reach one millimeter, which thus requires a membrane of the same order of magnitude. However, for such dimensions, the thermal mass, the mechanical hold, and the radiation loss of the membrane are such a problem that, in the end, they adversely affect the detector efficiency. Especially, a large heat capacity induces a high response time of the detector. Reinforcing the mechanical hold is not a satisfactory solution either, since a thick thermal insulation arm negatively affects the thermal insulation, and thus the detector sensitivity.
This is why, for such a frequency range, the radiation reception function is decoupled from the other functions. The receive function is thus provided by a planar antenna, and the function of conversion of the electromagnetic power into thermal power is provided by the resistive load of the antenna. The load dimensions conventionally fulfill the impedance matching conditions, which depend on the geometry of the antenna and on the nature of the layers supporting it, to obtain an optimal conversion. The resistive load is further in thermal contact with a thermometric element for the measurement of the generated thermal power. The assembly then forms a bolometer with an antenna.
Document US 2006/0231761, having its FIGS. 2 and 3a respectively reproduced in FIGS. 2 and 3, describes a bolometer 30 with an antenna, comprising a thermometric element 32 connected to a dipole-type antenna 38 via a resistive load 36. The assembly formed of the antenna, of the load, and of the thermometer is suspended above a substrate 34 by means of thermal insulation arms 39. The incident terahertz flow is thus detected by dipole antenna 38, which converts this flow into hyperfrequency surface currents, the generated currents inducing in return the heating of the resistive load 36, and thus of thermometric element 32.
The type of bolometer with an antenna however has two disadvantages. First, the antenna branches are separated by the bolometer. Now, the absorption efficiency of a bolometer with an antenna is maximum when the impedance of the resistive load is “matched” with the impedance of the antenna. More specifically, the impedance of an antenna comprises a real part, which is the resistance, and an imaginary part, which is the reactance, both variable according to the frequency of the current conducted by the antenna. There is a specific frequency, called “resonance frequency”, for which the resistance is maximum and the reactance is zero. The resistive coupling between the antenna and a resistive element, and thus the absorption efficiency of the bolometer, is optimal when the resistance of the resistive element is selected to be equal to the resistance of the antenna for the resonance frequency, or generally a resistance value at the resonance frequency ranging between 100 and 300 ohms.
Now, in the above-described architecture, the resistive load is itself coupled with the thermometric element, so that the general resistive element “seen” by the antenna is the combination of the resistive load and of the bolometer. In this case, a bolometer having a resistance “compatible” with the resistance of the antenna should thus be provided. However, bolometric materials efficient for thermometric detection at ambient temperature usually have a resistance greater than some hundred kΩ, or even greater than one MΩ, so that their impedance matching with the antenna is very low. Further, even though a bolometer (for example, of supraconductive type) would have a resistance “compatible” with that of the antenna, the very principle of a bolometer is to see its resistance vary along with temperature. Accordingly, for the very definition of the bolometer, it is impossible to have an optimal impedance matching for all temperatures observed with this type of architecture.
Another disadvantage resulting from this architecture is that it detects a radiation according to a single polarization axis, and that it is accordingly very sensitive to the polarization of the incident radiation. To detect an incident radiation of any polarization, at least two different polarization axes, advantageously orthogonal, should thus be defined. Now, the integration of a second dipole antenna, having a polarization axis different from that of the first antenna, in the bolometer with an antenna of document US 2006/0231761 is very difficult without strongly altering the detector performance, due to the presence of the thermal insulation arms.
Usually, two categories of antennas are used to obtain a bipolar detection, that is, on the one hand, circular polarization antennas, such as for example spiral antennas, and on the other hand, a system of two crossed antennas respectively sensitive to two orthogonal rectilinear polarizations, such as double bowties or double dipoles.
For the second category, to obtain an equal detection according to the two polarization axes, the crossed antennas should be symmetrical for the two orthogonal polarizations, which means that the physical size of the antennas should be identical whatever the polarization.
Now, this is difficult with a bolometric membrane such as described in relation with FIG. 1. Indeed, under the assumption that the antennas are placed on the suspended membrane, thermal arms 18, which thermally insulate the antennas and the thermometric element, impose a limit to the geometric length of the antenna in one of the two polarization directions since the antenna must not cross the two thermal insulation arms, which would very negatively affect the thermal insulation. Also, this symmetry constraint for the dipole antenna imposes a maximum size of the antenna equal to the distance between the two thermal arms. Such a technological approach thus adversely affects the advantage of forming a bolometric detector where a large antenna for coupling the submillimetric wave is associated with a small bolometric membrane. Indeed, the physical size of the antenna is always smaller than the size of the bolometric plate. Such a situation is incompatible with a detection in the spectral range, which requires large antennas for an efficient coupling.
To overcome the size limitation imposed by the thermal insulation arms, a solution is to transfer at least one of the antennas outside of the suspended membrane, for example, on the support above which the latter is suspended, and to provide a coupling mechanism which transfers the electromagnetic power received by the transferred antenna(s) to the suspended membrane by a capacitive coupling mechanism.
Such a solution is for example described in document US 2010/276597. Referring to FIGS. 4 and 5, this document describes a bolometer 40 which comprises an insulating substrate 42 having a first bowtie antenna 56 deposited thereon. A microbridge 50 is suspended above 10 substrate 42 by support and thermal insulation arms 54. A second bowtie antenna 44, crossed with first antenna 56, is further formed on microbridge 50 and is resistively coupled with a conductive layer 66 thereof. Fins 68, 70, 72, made of the same material as antenna 44 are also provided on conductive layer 66 with surfaces facing first bowtie antenna 56. Fins 68, 70, 72 are thus capacitively coupled with first bowtie antenna 56. A thermometric material 15 layer 74 is further deposited on an insulator layer 76 in contact with conductive layer 66.
A portion of the incident optical flow is thus collected by transferred antenna 56, which generates surface currents therein. By capacitive effect, the surface currents couple with fins 68, 70, 72. The latter thus form first antennas in microbridge 50.
However, capacitive coupling has, by nature, a lower performance than a resistive coupling, due to a lack of optimal matching. Indeed, when using a capacitive coupling between a “primary” transferred antenna and a “secondary” antenna in the microbridge, the value of the capacitance formed between the primary and secondary antennas adds to the reactance of the primary antenna. Since this reactance is no longer zero at the resonance frequency, the capacitive coupling has a lower performance than a resistive coupling with an matched impedance load. Further, since the capacitance of the capacitive coupling between primary antenna 56 and secondary antennas 68, 70, 72 varies according to frequency, the capacitance decreasing according to frequency, the impedance matching is not constant, and especially alters as the frequency decreases.
Another disadvantage of capacitive coupling is that it requires a large surface area of the antenna to have a certain efficiency. Indeed, the intensity of the capacitive coupling is proportional to the opposite metal surface areas, which thus implies significantly increasing the thermal mass of the microbridge and accordingly adversely affecting the detector response time.